The invention relates to laser range finders that are based on the principle of measuring a pulse transit or phase transit time.
Laser range measuring devices that are known in the prior art are based on measuring the transit time of the pulse or the transit time of the phase that can be distinguished from one another by a basic arrangement of the transmitting and receiving channels. This occurs by dividing these range finders in devices wherein the transmitting channel is arranged next to the receiving channel. Thus, the optical axes of these channels extend parallel to each other with a defined spacing from each other. These devices have transmitting and receiving channels that are arranged coaxially with each other, so that their optical axes coincide.
In the measurement of objects, the optical xe2x80x9ccross talkxe2x80x9d of the receiver which can be caused, by back scatter of dust articles at close range, can be optically reduced only by two measures: by reducing the receiver surface and by increasing the axial spacing between the transmitting channel and the receiving channel.
However, when the distance of the object is reduced, the effect of both measures is that the beam of measuring rays reflected by the object migrates away from the receiver more rapidly.
For range finding devices that are solely designed for close range viewing, these devices comprise coaxial transmitting and receiving channels. Thus, the transmitting lens, which also may also be an individual lens, represents the receiving lens as well. A beam divider is located within the focal length of this object, which produces the focal plane of the lens in two planes that are conjugated in relation to one another. The transmitter is located in these focal planes, on the one hand, and the receiver on the other. Thus, the measuring radiation emitted from the transmitter is collimated by the lens, reflected by the object, and is always reproduced on the receiver irrespective of the distance of the object.
This arrangement is suited for close ranges because of the relatively high intensity of the measuring radiation reflected back by the object to the receiver. Thus, the following characteristics are present:
the opening angle of the lens, optimized for emitting the beam of measuring rays, is adequate for receiving the reflected measuring radiation;
the dynamic range of the receiver is adjusted so that a reflection of the measuring radiation is not detected on dust particles; and
a loss of intensity on account of the beam divider poses no problems.
This arrangement is unsuitable for range finders designed to view in a far range because of the low intensity of the reflected measuring radiation of the desired object and the relatively high intensity of reflected radiation at close range. This result is caused by the reflection on optical structural elements such as the beam divider, the lens, and dust particles.
Therefore a parallel arrangement should be used for range finding devices for the far range, wherein the object to be measured is present at a distance that is infinite for the receiving lens, which also may be an individual lens. The measuring spot produced on the object to be measured, which is always coming from the infinite, is reproduced in the focus of the receiving lens. The transmitter and the receiver do not have to be arranged in two planes that are conjugated in relation to each other. This improvement permits the separation of the transmitting and receiving channels.
This arrangement is suitable for object distances in the far range because the relatively low intensity of the measuring radiation reflected by the object to the receiver. The following characteristics necessarily apply:
The opening angle of the receiving lens can be selected larger than the opening angle of the transmitting lens;
the dynamic range of the receiver can be adjusted so that a reflection of the measuring radiation on dust particles would be detected if these components of the radiation were to impact the receiver. This is avoided by the spacing between the optical axes of the transmitting and receiving channels and by a small receiver surface; and
a beam divider causes no additional loss of intensity.
This arrangement is unsuitable for measuring objects in the near range because of the resulting parallax, wherein as the distance becomes shorter, and the reproduction of the measuring spot increasingly migrates away from the receiver arranged on the optical axis of the receiving lens.
The designs described above may make it seem difficult to design a range finding device that is suitable for an extensive range of distance measurements, such as for objects to be measured both in the near and far ranges.
The demand for such range finders exists, for example in the construction industry, where a distance measuring range of from 0.3 to 30 m is important.
For range finding devices with a wide range finding spectrum, only an arrangement with parallel transmitting and receiving channels can be used due to the reduction in intensity and the optical xe2x80x9ccross talkxe2x80x9d occurring in connection with the coaxial arrangement.
Such arrangements are disclosed in EP 0 701 702 and DE 198 60 464.
With the laser range finder described in EP 0 701 702, two basically different solutions are offered so that the measuring spot is always reproduced on the receiver. In the present case the measuring spot is produced on the inlet surface, in the near range as well.
This result may take place, on the one hand, by letting the light conductor inlet surface trail transversely to the optical axis in accordance with the displacement of the reproduction position of the measuring spot. As stated above, no complete follow-up intentionally takes place along the optical axis because it was found that follow-up in the concrete reproduction position leads to over controlling on the part of the evaluation electronics. Thus, the dynamic range of the control electronics of the receiver is exceeded.
On the other hand, the light conductor inlet surface can be arranged in a fixed manner. In addition, with short object distances, an optical deflecting means should be arranged outside of the optical axis, so that the measuring rays incident upon the receiving lens are deflected toward the light conductor inlet area in an increasingly slanted manner. In addition, it is assumed that correct deflection by the reproduction optics is not important because no intensity problems exist with object distances in the near range. The second-mentioned variation offers the advantage that it can make do without mechanically movable elements in the receiving channel.
However, it has the drawback that it is hardly possible to adapt the signal level, to the dynamic range of the receiver. The signal level is the intensity of the measuring radiation reflected by the object and impacting the receiver.
The range of the distance measurement is limited by the sensitivity range (dynamics) of the receiver when it is assured that part of the measuring radiation reflected on the object impacts the receiver surface.
The radiation intensity impacting the receiver area is substantially determined by the following features:
the transmitting capacity;
the intensity loss across the length of the radiation path which is equal to twice the distance to the object; and
the aperture range effective in the given case, such as the proportion of the surface area of the receiving lens that becomes effective in the given case for reproducing the reflected measuring radiation.
In German Patent DE 198 60 464, a different aperture range is effective depending on the distance of the object to avoid over controlling of the receiver. This is accomplished with the help of a special design of the receiving lens.
The receiving lens is a modified receiving lens with two focal points on the picture side. These two focal points are produced so that the receiving lens comprises a primary lens area and a secondary lens area. The secondary area of the lens extends across the entire diameter of the receiving lens stretched in the vertical direction in relation to the transmitting axis, and has a shape that becomes narrower toward the transmitting axis.
The two areas of the lens are designed so that the receiver receives a reflection signal disposed within the sensitivity range of the receiver across its entire desired distance range. However, the manufacture of such a receiving lens is difficult and it is very costly. The solutions offered by the prior art are similar in that they effect the reproduction of the measuring radiation on the receiver with an intensity that is suitable for the receiver with the help of measures that influence the guidance of the beam and require either costly optical or additional mechanical subassemblies.
The invention is based on the problem of providing a laser range finder for large ranges of measurement, so that the reproduction of the measuring radiation on the receiver occurs through a special embodiment of the receiver itself, with an intensity that is suitable for the receiver.
The goal is to reduce the cost for the manufacture of the optics, the assembly, the adjustment and the testing.
Furthermore, it is also an object of the invention to provide a laser range finding device for large ranges of measurement with high measuring accuracy and measuring speed.
Moreover, another object is to make it possible for a laser range finder to be produced with a smaller size than those known in the prior art.
This problem is solved by providing a photodiode chip arrangement in the form of a receiver that comprises at least two active photodiode surfaces arranged in a straight line. The alignment of this arrangement is determined by the spacing xe2x80x9caxe2x80x9d between the axes of the transmitting and the receiving lenses, and whereby the active photodiode surface arranged closest to the laser light source is located with spacing xe2x80x9caxe2x80x9d from the laser light source.
As no optical deflecting means are required, the receiving lens can be designed with a very short focal length (about 15 to 25 mm) in the form of an individual lens or an individual sphere. This design, in turn, permits the device to operate with very small photodiode receiver surfaces, which leads to a reduction of the dark current as well as a reduction of the capacities of the photodiodes. A reduction of the dark current increases the overall sensitivity of the laser range finder. A reduction of the capacity of the photodiodes permits a proportional increase in the modulation frequency and thus an increase of the measuring accuracy and a reduction of the measuring time.
Other objects and features of the present invention will become apparent from the following detailed description considered in connection with the accompanying drawings, which disclose three embodiments of the present invention. It should be understood, however, that the drawings are designed for the purpose of illustration only and not as a definition of the limits of the invention.